Various methods for detecting the presence of an analyte in a sample of biological fluid through the use of immunochemistry have been described. In the so-called "sandwich" method, for example, a target analyte such as an antigen is "sandwiched" between a labeled antibody and an antibody immobilized onto a solid support. The assay is read by observing the presence and amount of antigen-labeled antibody complex bound to the immobilized antibody. In the competition immunoassay method, antibody bound to a solid surface is contacted with a sample containing both an unknown quantity of antigen analyte and with labeled antigen of the same type. The amount of labeled antigen bound on the solid surface is then determined to provide an indirect measure of the amount of antigen analyte in the sample.
Because these and other methods discussed below can detect both antibodies and antigens, they are generally referred to as immunochemical ligand-receptor assays or simply immunoassays.
Solid phase immunoassay devices, whether of the sandwich or competition type, provide sensitive detection of an analyte in a biological fluid sample such as blood or urine. Solid phase immunoassay devices incorporate a solid support to which one member of a ligand-receptor pair, usually an antibody, antigen, or hapten, is bound. Common early forms of solid supports were plates, tubes, or beads of polystyrene which were known from the fields of radioimmunoassay and enzyme immunoassay. More recently, a number of porous materials such as nylon, nitrocellulose, cellulose acetate, glass fibers, and other porous polymers have been employed as solid supports.
A number of self-contained immunoassay kits using porous materials as solid phase carriers of immunochemical components such as antigens, haptens, or antibodies have been described. These kits are usually dipstick, flow-through, or migratory in design.
Tom et al., U.S. Pat. No. 4,366,241, and Zuk, EP-A 0 143 574 describe migration type assays in which a membrane is impregnated with the reagents needed to perform the assay. An analyte detection zone is provided in which labeled analyte is bound and assay indicia is read.
Bernstein, U.S. Pat. No. 4,770,853, May et al., WO 88/08534, and Ching et al., EP-A 0 299 428 describe migration assay devices which incorporate within them reagents which have been attached to colored direct labels, thereby permitting visible detection of the assay results without addition of further substances.
Valkirs et al., U.S. Pat. No. 4,632,901, disclose a flow-through type immunoassay device comprising antibody (specific to a target antigen analyte) bound to a porous membrane or filter to which is added a liquid sample. As the liquid flows through the membrane, target analyte binds to the antibody. The addition of sample is followed by addition of labeled antibody. The visual detection of labeled antibody provides an indication of the presence of target antigen analyte in the sample.
Korom et al., EP-A 0 299 359, discloses a variation in the flow-through device in which the labeled antibody is incorporated into a membrane which acts as a reagent delivery system.
Baxter et al., EP-A 0 125 118, disclose a sandwich type dipstick immunoassay in which immunochemical components such as antibodies are bound to a solid phase. The assay device is "dipped" for incubation into a sample suspected of containing unknown antigen analyte. Enzyme-labeled antibody is then added, either simultaneously or after an incubation period. The device next is washed and then inserted into a second solution containing a substrate for the enzyme. The enzyme-label, if present, interacts with the substrate, causing the formation of colored products which either deposit as a precipitate onto the solid phase or produce a visible color change in the substrate solution.
Kali et al., EP-A 0 282 192, disclose a dipstick device for use in competition type assays.
Leuvering in U.S. Pat. No. 4,313,734 describes the use of gold sol particles as a direct label in a dipstick device.
Rounds in U.S. Pat. No. 4,786,589 describes a dipstick immunoassay device in which the antibodies have been labeled with formazan.
The requirements for washing steps and extended enzyme substrate incubation periods with dipstick immunoassay devices using enzyme labeled antibodies increases variability and thus the likelihood that minimally trained personnel and home users will obtain erroneous assay results.